1. Field of the Invention
This invention relates to an ink droplet jet device capable of stably jetting ink droplets at all times.
2. Description of the Related Art
Conventionally, a drop on-demand ink droplet jet device uses a piezoelectric ceramic element. This device is designed such that the volume of an ink channel is varied to jet ink in the ink channel through a nozzle when the volume of the channel is reduced and to introduce ink into the ink channel through an ink inlet port when the volume of the channel is increased. In this device, ink is jetted from a jetting device at a predetermined position according to print data to form desired characters or images.
Such an ink droplet jet device is disclosed in U.S. Pat. No. 5,016,028. The construction of this ink droplet jet device is described below.
As shown in FIG. 3, an ink jet printer head 1 comprises a piezoelectric ceramic plate 2, a cover plate 3, a nozzle plate 31, and a base plate 41.
The piezoelectric ceramic plate 2 is formed with plural grooves 8 by cutting the plate 2 with a thin disk-shaped diamond blade or the like. A side wall 11, which serves as a side surface for the grooves 8, is polarized in a direction as indicated by an arrow 5. These grooves 8 are designed to have the same depth and to be arranged in parallel to one another. The depth of the grooves 8 gradually decreases and becomes more shallow toward one end surface 15 of the piezoelectric ceramic plate 2 to form shallow grooves 16 near the end surface 15. Further, metal electrodes 13 are formed at upper half portions of both side surfaces of the inner surface of each groove 8 by a sputtering method or the like. In addition, metal electrodes 9 are formed at the side surfaces and the bottom surface of the inner surface of each shallow groove 16 by the sputtering method or the like. With this construction, the metal electrodes 13 formed at both side surfaces of the groove 8 are linked to the metal electrodes 9 formed at the shallow groove 16.
The cover plate 3 is formed of ceramic material or resin material. An ink inlet port 21 and a manifold 22 are formed in the cover plate 3 by polishing or cutting. The surface at the worked side of the groove 8 of the piezoelectric ceramic plate 2 and the surface at the worked side of the manifold 22 of the cover plate 3 are adhesively attached to each other through an adhesive agent from the epoxy group or the like (see FIG. 6). Accordingly, in the ink jet printer head 1, the grooves 8 are covered at the upper surfaces thereof to form plural ink channels 12 (FIG. 6) that are arranged in a lateral direction spaced from each other.
As shown in FIG. 3, a nozzle plate 31 provided with nozzles 32 at positions corresponding to the positions of the respective ink channels 12 is adhesively attached to the end surfaces of the piezoelectric ceramic plate 2 and the cover plate 3 through adhesive agent 33 from the epoxy group (see FIG. 4). This nozzle plate 31 is formed of plastic such as polyalkylene (for example, ethylene) terephthalate, polyimide, polyetherimide, polyetherketone, polyethersulfone, polycarbonate, cellulose acetate or the like.
The base plate 41 is adhesively attached to the surface of the piezoelectric ceramic plate 2, which is opposite to the worked side surface of the grooves 8, by an adhesive agent from the epoxy group (not shown). The base plate 41 is formed with conductive layer patterns 42 at the positions corresponding to the respective ink channels 12. The conductive layer patterns 42 and the metal electrodes 9 at the bottom surface of the shallow grooves 16 are connected to each other through conductive wiring 43 by a wire bonding method or the like.
Next, the construction of a control unit is described with reference to FIG. 5 showing a block diagram for the control unit. The conductive patterns 42 formed on the base plate 41 are individually connected to an LSI chip 51. Further a clock line 52, a data line 53, a piezoelectric line 54 and a ground line 55 are also connected to the LSI chip 51. On the basis of continuous clock pulses supplied from the clock line 52, the LSI chip 51 determines those nozzles 32 from which the ink droplet should be jetted according to data appearing on the data line 53. On the basis of this determination, a voltage V of the voltage line 54 is applied to the conductive layer patterns 42 conductively connected to the metal electrodes 13 of the ink channels 12 to be driven. Also, a voltage of 0 V of the ground line 55 is applied to the conductive layer pattern 42 conductively connected to the metal electrodes 13 other than the metal electrodes of the ink channels 12 to be driven.
Next, the operation of the ink jet printer head 1 is described referring to FIGS. 5 and 6. The LSI chip 51 determines that the ink should be jetted from the ink channel 12B of the ink jet printer head 1 based on print data. Upon this determination, a positive driving voltage V is applied to the metal electrodes 13E and 13F, and the metal electrodes 13D and 13G are grounded. As shown in FIG. 7, a driving electric field in the direction of arrow 14B occurs in the side wall 11B and a driving electric field in the direction of arrow 14C occurs in the side wall 11C. In this case, since both of the directions of the driving electric fields 14B and 14C are perpendicular to the polarization direction 5, the side walls 11B and 11C are rapidly deformed toward the inside of the ink channel 12B by piezoelectric thickness shear mode. This deformation causes the volume of the ink channel 12B to decrease and ink pressure to rapidly increase. Accordingly, a pressure wave is generated, which causes the jetting of the ink droplet from the nozzle 32 (FIG. 3) that is intercommunicated with the ink channel 12B.
Upon termination of the application of the driving voltage V, the side walls 11B and 11C gradually return to their initial positions before deformation (see FIG. 6), so that the ink pressure in the ink channel 12B is gradually reduced. This causes the ink to be supplied from the ink inlet port 21 (FIG. 3) through the manifold 22 (FIG. 3) into the ink channel 12B.
In the conventional ink droplet jet device as described above, sufficient volume variation is required for the ink channels 12 because the volume of the ink channels 12 is varied to jet ink droplets by the deformation of the side walls due to the piezoelectric thickness shear mode. To satisfy this requirement, as shown in FIG. 8, the upper surfaces of the side walls 11 and the cover plate 3 must be completely fixed to each other by the adhesive agent 4. That is, if the agent 4 has a small Young's modulus or has a thick adhesive layer, so that the elasticity of the adhesive layer of the agent layer 4 is large and its rigidity is small, then the adhesive agent 4 would be deformed in the direction opposite to the deformation direction of the side walls 11 as shown in FIG. 9. In this case, insufficient volume variation of the ink channels 12 occurs and a desired jet velocity of the ink droplet is not obtained. Further, solving this problem also causes a problem in the elasticity of the agent 33 for adhesively fixing the nozzle plate 31, the piezoelectric ceramic plate 2 and the cover plate 3. That is, if the adhesive agent has small elasticity, that is, a large rigidity, the end surfaces of the side walls 11 and the nozzle plate 31 will be firmly fixed. Thus, the volume variation of the ink channels 12 by the piezoelectric thickness shear mode of the side walls 11 will not be sufficiently carried out. In this case, the jetting of the ink droplet is adversely affected, and a desired jet velocity of the ink droplet cannot be obtained or no ink droplet is jetted.